1. Technical Field
The present invention relates to motor vehicle brake systems and more particularly to a corrosion immune anti-lock brake system exciter/tone ring for a wheel hub in a drum brake system or a disk rotor.
2. Description of the Problem
Brake units for motor vehicles should provide smooth braking with reasonable service life. Friction brakes have long been used due to their ease of construction, reliability and consistent performance. Some of the components in such brakes and related portions of the vehicle's wheels have been made from relatively inexpensive gray iron castings. Gray iron is however highly susceptible to corrosive attack, particularly in the operating environment of vehicles where brake components are open to the air, subject to substantial transient heating and exposed to water and salt water spray. One application of gray iron has been to disk rotors. In regular use, the working surfaces of the disks are rubbed clean by contact with the disk pads, which are typically made of a composite material and which rub off corroded areas of the disk. However other areas of the brake disks are not swept by the brake pads and thereby cleaned of corrosion and oxidation. Similarly wheel hubs on vehicles equipped with drum brakes have been subject to corrosive and oxidizing attack. Prior to anti-lock braking systems, such concerns were not paramount because the surface areas of the structures subject to attack were not critical to brake function or wheel integrity.
With the advent of anti-lock braking systems surface integrity of wheel components can take on importance. For disk brake systems typically a portion of the disk rotor has been cast with, or machined to exhibit, a surface contour allowing the rotor to serve as an exciter ring for the ABS system. The exciter ring is a cylindrical section of the rotor having a common axis of rotation with the wheel and which is shaped as a series of teeth positioned in a ring, flat in the plane of rotation of the wheel and oriented to pass closely by a stationary magnetic sensor. The stationary sensor is a variable reluctance sensor which generates an electrical pulse train as a function of the varying magnetic flux leakage between the sensor head and the exciter ring. The frequency of the resulting electrical pulse train indicates the rotational speed of the wheel and large non-continuous changes in wheel rotational speed are taken as indicating skidding of the wheel. The generation of clean pulse train is greatly aided by having teeth of uniform shape and size so that the spacing of the teeth from and the surface area present by the teeth to the sensor is uniform. Corrosion or oxidation of the iron forming on the surfaces of the teeth compromises these factors, resulting in difficulty in detecting the passage of teeth by the sensor. This in turn can make it difficult to qualify pulses and possible corruption of the resulting pulse train.
Corrosion protection coatings have been proposed for use with cast iron exciter rings. Such coatings are described in U.S. Pat. No. 5,569,543. Other corrosion inhibiting coatings have been supplied by Magni Corp., including the Magni 109 and Magni 111 coatings. Such coatings can be easily compromised when applied to exciter rings since the sensor usually needs to pass within very close proximity to the teeth and, consequently, the chance exists for removal of the coating from the teeth, which again leaves the teeth exposed to corrosive agents or oxidation. In addition, such coatings are relatively expensive.
The problem of corrosion and its implications for vehicle ABS systems manifests itself somewhat differently with drum brake equipped vehicles. For drum brakes the exciter ring has not been an integral part of any part of the working brake, but rather has been a separate part press fitted on the end of a wheel hub. Press fitted rings can be made of ferro-magnetic materials less susceptible to attack that iron, such as steel, however, even these materials are not immune from such problems.
Japanese Patent Publication 2005121669 discloses a tone ring for a rolling bearing unit where powdered ferromagnetic material is dispersed in a composite material such as rubber and elastomer.